1. Field of the Invention
The present invention concerns an electric injection mechanism for assuring driving of the screw forward by an electric motor.
2. Detailed Description of the Prior Art
The electric injection mechanism, having an electric motor such as a servomotor or the like as a driving source, for forward driving of a screw by converting rotation movement by the electric motor into linear movement by a ball screw shaft and a ball nut member, includes one adopting a single ball screw shaft and one adopting a plurality of ball screw shafts.
In any of these types, a driver for driving a screw forward is provided between a pair of front and rear support plates on a machine base, by inserting the driver onto a guide bar, putting the driver slidably on an installation member, or the like. As the ball screw shaft rotated by the electric motor engages with the ball nut member installed inside the screw driver, the axial lines of the ball screw shaft and the ball nut member are misaligned relatively due to the attachment error of the ball screw shaft or lack of rigidity of the mechanism, easily causing a radial force or moment.
In an electric injection mechanism adopting a single ball screw shaft, the ball screw shaft fixation point is only disposed at the rear end born by the rear plate, and there is a flexibility at the meshing region with the ball nut member; therefore, it is thought that a radial force due to a relative misalignment of axial lines, or a gnawing force by the generation of a moment is hardly caused.
However, in reality, the gnawing force is caused by the relative misalignment of the axis lines of the ball screw shaft and the ball nut member due to the fact that the screw driver is supported by inserting it onto the guide bar and that the rotatable ball screw shaft of the rear support p plate engages by threads with the ball nut member installed inside at the rear middle of the screw driver by rotating the ball screw-shaft through the motor, and the screw driver is driven forward together with the screw.
Further, in an electric injection mechanism adopting a plurality of single ball screw shafts and ball nut members, as a pair of ball screw shafts are disposed rotatably in parallel with the guide bar, and as the ball nut members disposed through the right and left of the screw driver engage by threads with the ball screw shafts, the front and rear ends of the ball screw shafts are in a state fixed to the front and rear plates. As the screw driver is in a state supported through insertion of both the guide bars and ball screw shafts, the flexibility to the radial force or moment is inferior to the case of the single ball screw shaft. If the parallelism between the guide bars and the ball screw shafts deteriorates even slightly, an excessive gnawing force is caused in the engaged region, and a premature wear, peeling off or other phenomena is easily caused in the ball screw.
Additionally, a bending generates in the ball screw shaft and the ball screw shaft comes to be supported slantingly by a bearing. A problem of premature deterioration of the bearing member thereby occurs.
Still further, in an electric injection mechanism linking front and rear guide plates by guide bars, a deflection may be generated in the front plate and the guide bar and may deform the mechanism on the advance and backward movement during the operation, whereby the ball screw shaft bends in a quadratic curb shape between the front and rear fulcrum points. The load distribution on a ball and a ball screw groove in the ball nut member becomes uneven. An inclination generated causes an excessive gnawing force in the ball screw shaft and bearing portion and in the engagement region with the ball nut member, and the ball screw shaft may wear or deteriorate abnormally, and further even break by the stress concentration at this moment.
The present invention, devised to resolve the aforementioned problems in the ball screw shaft of the driving means adopted by the electric injection mechanism, has an object to provide a novel electric injection mechanism allowing the reduction of the generation of an excessive gnawing force, by installing a stress relaxation means by an annular groove in the ball screw shaft.
According to the above object, one embodiment of the present invention comprises a screw driver disposed movably between a pair of front and rear support plates on the machine base, a driving means comprising a ball screw shaft and a ball nut member disposed over the screw driver and the support plate, and an electric motor of the driving means. A shaft between a ball screw portion and a bearing portion of the ball screw shaft is partially formed with a diameter smaller than a root diameter of a ball screw groove, and a stress relaxation groove having a bottom face with a flat concave curbed face is formed in the shaft.
In addition, another embodiment of the present invention comprises a pair of front and rear support plates disposed on the machine base by linking by means of a plurality of guide bars, a pair of right and left ball screw shafts disposed rotatably in parallel with the guide bars over the support plates, a screw driver provided inside on the right and left with a pair of ball nut members meshing with the ball screw shaft and inserted movably into the guide bar between the support plates, and an electric motor of a driving means comprising the ball screw shaft and ball nut member. A shaft between a ball screw portion and a bearing portion of the ball screw shaft is partially formed with a diameter smaller than the root diameter of a ball screw groove, and a stress relaxation groove having a bottom face with a flat concave curbed face is formed in the shaft.
Further, another embodiment of the present invention comprises a pair of support plates disposed in front and behind an installation member on a machine base, a pair of right and left ball screw shafts juxtaposed rotatably over the support plates, a screw driver provided inside on the right and left with a pair of ball nut members meshing with the ball screw shaft, and put slidably on the installation member between the support plates, and an electric motor of a driving means comprising the ball screw shaft and ball nut member. A shaft between a ball screw portion and a bearing portion of the ball screw shaft is partially formed with a diameter smaller than a root diameter of a ball screw groove, and a stress relaxation groove having a bottom face with a flat concave curbed face is formed in the shaft.
In the aforementioned configurations, the annular stress relaxation groove installed around the shaft portion of the ball screw shaft reduces an excessive gnawing force due to the generation of a radial force or moment, or an excessive gnawing force that tends to be generated due to the deflection during the moment of advance or regression operation of the injection mechanism, and resolves an excessive load at the engagement region of a ball screw portion with the ball nut member. The stress relaxation groove thereby prevents an abnormal wear or deterioration and further breakage of the ball screw shaft, maintains a smooth advance and regression operation of the screw driver, and extends the service life of the ball screw portion and the ball nut member.